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October 19, 2003
Colin at sprite parts wrote
I happen to have a brand new 12G295 head, still in the BMC wax paper. So I measured it for you. It is exactly 2.754" thick.
So,
That means it's just inside the drawing tolerance of 2.75" +/- 0.005"
However,Colin
The depth dimension of the combustion chamber - could you give us a dimension of this please. ,( which is more critical to find out out if the engine face has been machined)
In fact any dimensions for a brand new head would be useful.
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I include part of an email I sent recently to another lister which might be useful regarding skimmiing the head to increase CR.
Reference to " how much can I machine off the head to raise CR, without causing damage to the head"
I think the problem was not the valve smacking into the piston syndrome but more that there is a drilled rocker shaft oil feed close to the cyl. heads surface by Nr 1 chamber.
Machine too much off and this oil channel becomes exposed. This however, can be remedied by milling a groove then brazing in some 3/16"OD bundy tube and remachining flush.
I quote " as all the castings vary, it is impossible to lay down a fixed
maximum amount of metal that can be safely machined off.
... standard heads, (except cooper S and mini 1275GT) are 2 3/4" deep +/- 0.005"
.... Some heads will withstand metal removal of up to 0.125" without distorting.
(cooperS recommended maximum =0.060")
And don't forget for tuning the head , the kidney shape is reprofiled, the valve guide is tapered and the platform for the exhaust valve guide is chamfered to aid flow, so with all that metal removed you have to skim the head to bring the compression back up.
9:1 is about max for a road car but you should fit higher tensile studs and torque them to a higher value than standard, also the each chamber has to be identical in cc to its neighbour.
Bob, I took 40 thou' (1mm) off my 1098 head years ago, fitted with flat top pistons and it ran very well. I had stock clearances and throw on the valves but I was using leaded fuel then.
It was 12G295 casting, originally fitted to 998cc mini coopers, MG1100 and mk 3 sprites
(I had used it on an MG 1100 transverse engine fitted in a BL mini )
rgds
Alan
"alan.east-jones" < alan.east-jones@... > 26214
Posted on October 19, 2003 11:57 AM
Posted to category(s) Carburetor | Cylinder Head | Drive Train | Engine (external) | Engine (internal) | Exhaust System | Exterior | Fuel System | Gauges | Lubrication System | Miscellaneous | Restoration | Technical Info | Tuning
